Changes in orogenic style and surface environment recorded in Paleoproterozoic foreland successions

The Earth’s interior and surficial systems underwent dramatic changes during the Paleoproterozoic, but the interaction between them remains poorly understood. Rocks deposited in orogenic foreland basins retain a record of the near surface to deep crustal processes that operate during subduction to collision and provide information on the interaction between plate tectonics and surface responses through time. Here, we document the depositional-to-deformational life cycle of a Paleoproterozoic foreland succession from the North China Craton. The succession was deposited in a foreland basin following ca. 2.50–2.47 Ga Altaid-style arc–microcontinent collision, and then converted to a fold-and-thrust belt at ca. 2.0–1.8 Ga due to Himalayan-style continent–continent collision. These two periods correspond to the assembly of supercratons in the late Archean and of the Paleoproterozoic supercontinent Columbia, respectively, which suggests that similar basins may have been common at the periphery of other cratons. The multiple stages of orogenesis and accompanying tectonic denudation and silicate weathering, as recorded by orogenic foreland basins, likely contributed to substantial changes in the hydrosphere, atmosphere, and biosphere known to have occurred during the Paleoproterozoic.

of the NCC, which led to the outward growth of the NCC and final incorporation into the Columbia supercontinent 46,47 .Thus, the core debate focuses on 1) whether there was a late Archean collision between continental blocks or between arcs and microcontinents 44,45 or not 9 , and 2) the nature of the early Paleoproterozoic evolutionary process from 2.5 Ga to 1.8 Ga 6,44 .In addition, some general and frontier scientific questions regarding the styles of orogenesis and plate tectonics require further constraints from different perspectives 43,48 .The sedimentary archives from orogenic forelands record erosion and sedimentation of detritus (including various magmatic and metamorphic minerals) from adjacent orogenic crust, thus providing an excellent window to solve the above-mentioned issues.
The Songshan Group unconformably overlies the Archean basement in the southern NCC and has been interpreted as either a post-cratonisation sedimentary succession developed along the cratonic or rift margin 39 , a retro-arc foreland basin developed behind an Andean-type continental arc 40,49 , or a peripheral to retro-arc foreland basin 2,31 .The latter two models similarly interpreted the succession as a result of the eastward subduction of oceanic lithosphere beneath the Proto-Eastern Block, although the depositional ages and deformational history of the Songshan Group remain incompletely constrained [49][50][51] .
In this study, we focused on the deposition and deformation of the Songshan Group, and conducted detailed structural investigations and multi-mineral geochronological analysis.These datasets enable us to better constrain its depositional-to-deformational evolution and characterise the two orogenic styles in the NCC during the Neoarchean to Paleoproterozoic transition from sedimentary and deformational perspectives that have been largely underestimated in the past.

Zircon and rutile U-Pb geochronology and trace elements
A total of eight samples were selected for zircon U-Pb dating and four samples containing rutiles were selected for U-Pb dating (see Methods and Supplementary Data 1).The results of the zircon and rutile analyses and the maximum depositional ages are described below.Three age criteria, including YSG (youngest single grain), YC1σ (youngest cluster overlapping at 1σ uncertainties) and YC2σ (youngest cluster overlapping at 2σ uncertainties), were calculated and used to characterise the maximum depositional ages [52][53][54] .The MDA is listed in Supplementary Table S1.

Trace element characteristics
The rare earth element (REE) compositions of zircon grains from the Dengfeng Complex and Songshan Group are distinct from those of the Wufoshan Group (Supplementary Fig. 8), which can provide useful constraints on their source regions.For the zircons from the Dengfeng Complex and Songshan Group, the majority are characterised by enrichment of heavy REE (HREE), with relatively high (Lu/Dy)N ratios and Lu values (Fig. 6).
For zircons from the lower Wufoshan Group, some have low (Lu/Dy)N ratios and Lu values (Fig. 6), reflecting high-pressure metamorphism associated with garnet (metamorphic garnet signature, MGS) in their source regions.In addition, most analysed zircon rims with metamorphic ages clustering at 2.1-1.85Ga have enriched HREE, which may be attributed to metamorphism under high-T granulite or amphibolite facies (Supplementary Fig. 8).Therefore, the style of Paleoproterozoic metamorphism recorded by detrital zircons is distinct from that of the Neoarchean.This result matches 1) the global detrital zircon dataset, which shows a similar age peak of MGS zircon in the Paleoproterozoic 55 (Fig. 6); and 2) more abundant high-pressure metamorphic crustal rocks in this time period 56,57 (Fig. 1a).

Rutile
Rutile grains from the metaconglomerate (20SS09) and quartzites (20SS01, 20SS05) of the lower Songshan Group are mostly euhedral or subhedral, and are generally aligned to the foliations, with lengths of ~40-130 μm and length/width ratios from 1:1 to 2:1 (Supplementary Fig. 6).Rutile records yield U-Pb ages of ca.2.50 Ga (n=3), 2.31-2.05Ga (n=11) and 2.0-1.75Ga (n=62), with two peaks at 1.95 Ga and 1.87 Ga (Fig. 5e).This peak age of rutile is significantly younger than that of detrital zircon from the same rocks, together with the euhedral to irregular rutile shape and alignment with the foliation, indicating that the majority of young rutile populations (e.g., younger than the upper age limit of deposition--2.30Ga--of the lower Songshan Group) are of metamorphic origin.This interpretation is supported by other lines of evidence.For instance, pressure-independent Zr-in-rutile geothermometry 58 yields temperatures (469-586 °C, mean 505 °C, n=44) for <2.0 Ga rutiles (Fig. 7, Supplementary Table S2), consistent with estimate of the metamorphic temperature of the lower Songshan Group (see Supplementary Note 4 below).Minor older detrital rutiles (ca.2.50 Ga) and the metamorphic temperature lower than rutile Pb closure temperature (~600 °C) support that the rutile U-Pb isotope has not been reset.

Rutile from quartz sandstone sample 20DF03 of the Ma'anshan Formation of the lower Wufoshan
Group are mostly rounded, with lengths of ~60-150 μm and length/width ratios from 1:1 to 2:1 (Supplementary Fig. 6).The rutile morphology, along with the unmetamorphosed host quartz sandstone, indicates their detrital origin, thus recording metamorphic information of their source regions.The analyses are dominated by Paleoproterozoic ages, with the main populations (n=54) clustering at 1.94 Ga and 1.83 Ga, and minor peaks at 2.32 Ga and 2.15 Ga.The YSG, YC1σ and YC2σ ages are 1746 ± 28 Ma, 1770 ± 10 Ma (n=10) and 1802 ± 6 Ma (n=23), respectively.Relative to rutile from the Songshan Group, rutile from the Wufoshan Group yields a much higher average Zrin-rutile temperature for <2.0 Ga rutile (~577-876 °C, mean 747 °C, except for one 479 °C, Fig. 7, Supplementary Table S2).

Analytical method
The 40 Ar/ 39 Ar analyses were conducted at the Western Australian Argon Isotope Facility at Curtin University, Australia.The selected amphibole and mica grains were leached in diluted (5N) HF for one minute and then rinsed using distilled water in an ultrasonic bath.After rinsing, minerals were loaded into aluminium discs that were Cd-shielded to minimize undesirable nuclear interference reactions, and irradiated for 40 h in the OSU TRIGA nuclear reactor (Oregon State University, USA), in a central position.The J-values calculated from the GA1550 standard grains within the surrounding pits yielded a value of 0.01096.
The amphibole and mica grains were step-heated using a continuous 100 W PhotonMachine© CO2 (IR, 10.4 µm) laser fired on the crystals for 33 seconds.Each standard crystal was fused in a single step.The gas was purified in an extra low-volume stainless steel extraction line of 240cc, using two SAES AP10 and one GP50 getter.Ar isotopes were measured in static mode using a low volume (600 cc) ARGUS VI mass spectrometer from Thermofisher© set with a permanent resolution of ~200 59 .We measured the relative abundance of each mass simultaneously using 10 cycles of peak-hopping and 33 s of integration time for each mass.The raw data were processed using the ArArCALC software 60 .
The relative abundance of the Ar isotopic data is provided in Supplementary Data 3 and has been corrected for blanks, and mass discrimination.
The metamorphic P-T estimates are referred to ref 42 , which are consistent with high greenschist to amphibolite facies metamorphism with garnet-bearing rocks preserving two generations of zoning 42,61 .

Single mineral geothermometer
The Songshan Group mainly consists of thick quartzites and quartz mica schists, with greenschist and locally lower amphibolite facies metamorphic grades.The mica quartz schists in the Songshan Group are foliated, with a mineral assemblage dominated by quartz, mica (mainly muscovite with minor biotite), feldspar (mainly K-feldspar and albite), and minor zircon, magnetite, rutile, and apatite (Supplementary Fig. 3f-g).Due to the lack of garnet in the Songshan Group, little appropriate mineral geothermobarometer can be used to estimate the P-T conditions, except for single mineral geothermometers like Ti-in-biotite/muscovite geothermometers.The Ti-in-biotite (TiB) geothermometer 62 is calibrated for TiO2-saturated, rutile-and/or ilmenite-bearing metapelites under wide P-T (450-840 °C/1-19 kbar) and compositional ranges, with a random error of ± 65 °C.The mineral chemical composition was analysed using electron microprobe JEOL JXA8230 at the School of Earth Sciences, China University of Geosciences, Wuhan, following the same procedure provided by ref 63 .The analytical error is ± 2 wt.%.
The mean temperature (526-586 °C, assuming 4 kbar) of quartz mica schists obtained by the TiM geothermometer from Group 1 muscovite is in agreement with the temperature (average 558-598 °C at 4 kbar) obtained by the Ti-in-biotite geothermometer (Supplementary Table S3).In addition, the Zrin-rutile geothermometer yields similar temperature ranges (469-586 °C, average 505 °C) for <2.0 Ga metamorphic rutile from quartzites (Supplementary Table S2), implying that the estimation of the metamorphic temperature of the Songshan Group to be ~470-600 °C is reasonable.The peak metamorphic temperature (<600 °C) is lower than the closure temperature of the rutile Pb isotopic system 65 , indicating rutile might have continuously grown and been preserved during prolonged orogenesis (from thickening to cooling) from ca. 2.0 Ga to 1.8 Ga.

Phase equilibria modelling
In order to investigate the P-T range of the observed mineral assemblage, we further conducted phase equilibria modelling using GeoPS 66 , with the internally consistent thermodynamic datasets of ds62 67 , a chemical system of MnNCKFMASHTO, and the mineral activity-composition (a-x) model following ref 68 .The bulk composition is based on XRF analysis.The H2O is based on LOI, and Fe 3+ /åFe 2+ is set to 0.1.The detailed procedure followed ref 42 .
The phase diagram of a representative quartz mica schist sample (20SS06) shows that rutile is stable over a large P-T range below the solidus (Supplementary Fig. S11), suggesting that rutile can possibly form and/or preserve during greenschist to amphibolite facies conditions in such rocks.The characteristic mineral assemblage (quartz-muscovite-K-feldspar-albite-rutile, Supplementary Fig. S3g) is stable under a large field of <620 °C/<12 kbar as constrained by the absence of garnet, plagioclase and melt (Supplementary Fig. S11).The precise constraint on P-T conditions (in particular P) is difficult due to the lack of garnet and low confidence about the intersection of other mineral composition isopleths.For instance, the Si of white mica in sample 20SS06 ranges from 3.32 to 3.44 c.p.f.u.(based on 11 oxygen atoms), which would yield relatively high pressure up to 7.6-9.2kbar at a rutile ZIR temperature of ~505 °C.If considering the absence of plagioclase and garnet in this sample, then a relatively wide metamorphic pressure range of ~3.6-9.2 kbar can be fairly inferred based on the intersection between the plagioclase-and garnet-in lines and the Zr-in-rutile temperature.
Collectively, we infer that our samples from the lower Songshan Group may have undergone greenschist to lower amphibolite facies metamorphism, with a wide P-T condition of 470-600 °C/~3.6-9.2 kbar.Considering the different burial levels of the crust during orogenesis, lower or higher P and/or T conditions may be expected for other rocks of the Songshan Group within lower-or higher-strain zones.Note: WMA--weighted mean age.Due to the uncertain pressure of detrital and metamorphic rutiles, the Zr-in-rutile geothermometer follows pressure-independent calibration by Ferry and Waston 58 , which yields temperatures comparable with those by pressure-dependent calibration (aquartz) by Kohn 73 in the most temperature range when considering a pressure 13 ± 5 kbar with error.Note: QMS--quartz mica schist.The temperature by Ti-in-biotite (TiB) geothermometer 62 is preferred, while the temperature by Ti-inmuscovite geothermometer 64 is used for reference and comparison.

Complex. b
Folds within the Wuzhiling Formation.c Large isoclinal overturned tight folds within the Wuzhiling Formation.d Boudinaged and fragmented quartzite layers within highly-deformed quartz-mica schist in the Wuzhiling Formation.e Top-to-the-SE sense of thrusting marked by the asymmetric quartzite boudinage and small-scale folds.from the Luohandong Formation of the lower Songshan Group.e, k quartzite sample 20SS05 from the Wuzhiling Formation of the lower Songshan Group.f quartz mica schist sample 20SS06 from the Wuzhiling Formation of the Songshan Group.g quartzite sample 21SS25 from the Huayu Formation of the upper Songshan Group.h, l quartz sandstone sample 20DF03 from the Ma'anshan Formation of the Wufoshan Group.

Table 2 .
Summary of rutile ages and Zr-in-rutile temperatures.